1. Field of the Invention
The present invention relates to communication systems using narrow or very narrow band channels, so that the number of channels available for different types of communication systems (for example, cellular telephones, pagers, police/fire/rescue) can be increased. Specifically, the invention relates to a system and method for transmitting and receiving PM- or FM-based signals on narrow or very narrow band channels.
2. Related Art
In recent years, the VHF and UHF bands that were reserved to be allocated for use in communications have become highly congested, especially with the expanding use of cellular telephone and pager networks. However, due to restrictions imposed by current technology, government regulation and international agreement, the total available bandwidths in the VHF and UHF ranges, and consequently, the number of channels available in those band widths, are limited. With an expanding demand for communication systems which use those frequency bands, there is a growing need for a larger number of channels to be made available for allocation.
This need for more channels has led many manufacturers in the field to use higher frequency bands to avoid interference between channels and to achieve better frequency management. However, it is believed that using the higher UHF bands will expose users and operators to high frequency radiation that may cause health problems.
Another solution to the need for more channels which avoids the potential health risks involved in using the higher UHF bands is to increase the number of channels within the currently used frequency bands using a system with narrow channel bandwidths. However, such systems as currently designed produce poor sound quality and low data transmission rates.
Thus, there is a need in the art to provide transmission systems in which the information is transmitted in a narrow or very narrow bandwidth channel to provide a greater number of channels, but which still provides good sound quality at the receiving end.